Lightning discharges are natural phenomena that are able to generate energies up to several GJ (Rakov & Uman, 2006). The transient radio waves emitted by the lightning discharges are called "atmospherics", or "sferics" for short. These electromagnetic waves widely cover the frequency spectrum from ∼1 Hz to ∼300 MHz, with relatively large spectral amplitudes in the extremely low frequency (ELF) range and very low frequency (VLF) range and a relative maximum at ∼10 kHz (e.g., Burke & Jones, 1992;Taylor, 1960;Weidman & Krider, 1986). Another source of VLF electromagnetic waves is radio transmitters (Barr et al., 2000).Lightning discharges fall into two major categories: cloud-to-ground discharges (CGs) and in-cloud discharges (ICs), where the CGs exhibit much larger peak currents compared to the ICs (e.g., Betz et al., 2009;Fiser et al., 2010). Cloud-to-ground discharges can lead to serious hazards and are more relevant to human life than ICs. The peak current of CGs has been studied for the purpose of lightning protection (e.g., Chowdhuri et al., 2005;Schulz et al., 2016;Takami & Okabe, 2007;Visacro, 2004), and existing lightning location networks mainly aim at detecting cloud-to-ground discharges with the time-of-arrival (TOA) technique.The ionosphere (σ ≈ 10 −4 Sm −1 − 10 −2 Sm −1 ) and the earth's ground (σ ≈ 10 −3 Sm −1 ) form a natural wave guide with large conductivity boundaries, while the atmosphere in between exhibits a much lower conductivity. This wave-guide is able to guide the propagation of sferics (e.g.

Optical observations of lightning discharges are often compared to their electromagnetic signatures at radio frequencies, which are either recorded on board satellites or on the ground (e.g.,

<div> <div> <div> <p>Recently, Transient Luminous Events (TLEs) in the mesosphere and lightning activity near thunderstorm tops have attracted great interest. The Atmosphere-Space Interactions Monitor (ASIM) and the Modular Multispectral Imaging Array (MMIA) are on board the International Space Station (ISS) to record the lightning activity and TLEs in the UV band (180-230 nm) as well as the blue (337 nm) and the red (777.4 nm) emissions (Chanrion et al. [2019], Neubert et al. [2019]). Blue luminous events recorded by ASIM during the nighttime were first reported by Soler et al. [2021].</p> <p>During 23:00-23:05 UTC on 3rd, February 2019, 188 MMIA triggers were recorded and more than 2000 lightning strokes were reported by the lightning detection and location network. We focus on a blue discharge event that happened at 23:02:41 UTC, which was caused by a negative narrow bipolar event (NBE) with no red and UV photomultiplier tube (PMT) pulses associated with it. The novelty of this work is that the height determination is carried out by using the ground-based electric field measurements and the space-based optical measurements from ASIM. The low-frequency electric field receiver was set up in Carnarvon, 30.97° S, 21.98° E, South Africa. The blue discharge height (15.83-18.67 km), calculated using the electric field measurements, is derived from the skywaves arrival times with a spherical Earth model. The ionospheric height calculated by this model (93.89 km) is consistent with that determined by the averaged cloud to ground discharges waveforms (93.68 km). The rising edge of the blue optical emission is analyzed to do the altitude estimation (14.3-15.8 km). The cloud top height is calculated as a reference (15.75-16.65 km), which is inferred from radiometric measurements, typically at a wavelength around 10 μm. The height of NBEs is important to help to understand the chemistry effects at the tropopause level caused by such events.</p> <p>In the future, this data set would be used to study other properties of many events such as blue events and red events.</p> <p> </p> <p>References</p> <p>Chanrion, O., Neubert, T., Lundgaard Rasmussen, I. <em>et al.</em> The Modular Multispectral Imaging Array (MMIA) of the ASIM Payload on the International Space Station. <em>Space Sci Rev</em> 215, 28 (2019). https://doi.org/10.1007/s11214-019-0593-y</p> <p>Neubert, T., Østgaard, N., Reglero, V. <em>et al.</em> The ASIM Mission on the International Space Station. <em>Space Sci Rev</em> 215, 26 (2019). https://doi.org/10.1007/s11214-019-0592-z</p> </div> </div> </div><div> <div> <div> <p><span>Soler, S.</span>, <span>Gordillo-Vázquez, F. J.</span>, <span>Pérez-Invernón, F. J.</span>, <span>Luque, A.</span>, <span>Li, D.</span>, <span>Neubert, T.</span>, et al. (<span>2021</span>). <span>Global frequency and geographical distribution of nighttime streamer corona discharges (BLUEs) in thunderclouds</span>. <em>Geophysical Research Letters</em>, <span>48</span>, e2021GL094657. https://doi.org/10.1029/2021GL094657</p> </div> </div> </div>

<p>Traditional long-range lightning detection and location networks use Time-of-Arrival (TOA) differences, and a single timestamp to locate lightning events. For long propagation distances, the amplitude of ground waves decays faster with distance than sky waves as a result of the ground conductivity and the effects of Earth curvature (Caligaris et al., 2008, Cooray, 2009, Hou et al., 2018). This can lead the skywaves to interfere with their large amplitudes when locating lightning.</p> <p>Coherency, which is short for phase coherency of the analytic signal, is used here, which exhibits lightning characteristics (Bai & Fullekrug, 2022). This work introduces a simulation study to lay the foundation for new lightning location concepts. A novel interferometric method using coherency is presented here, which expands the use of more data points of recorded lightning sferics to map the lightning into an area in a long-range network. In this map, each pixel corresponds to a lightning location with different coherency and time of arrival differences, simulated by shifting the complex lightning waveforms. In long-range networks, the coherency of the 1st skywave is larger than the ground wave, and it is difficult to distinguish them due to the short time delay between them. One solution is to use a small network so that the recorded waveforms are associated with short propagation distances which can eliminate the interferences caused by the first skywave. Another solution is to filter the data such that a lightning waveform is represented by an impulse. In this case, only one maximum coherency area exists for each event at the lightning occurrence time.</p> <p>In the future, the data collected with a real-time lightning detection network will be analysed to map the lightning events using the complex interferometric method for use in long-range lightning location networks.</p> <p> </p> <p><strong>References</strong></p> <p>Bai, X., & Füllekrug, M. (2022). Coherency of Lightning Sferics. Radio Sci., 57(5), e2021RS007347. doi: 10.1029/2021rs007347</p> <p>Caligaris, C., Delfino, F., & Procopio, R. (2008). Cooray–Rubinstein Formula for the Evaluation of Lightning Radial Electric Fields: Derivation and Implementation in the Time Domain. IEEE Trans. Electromagn. Compat., 50(1), 194-197. doi: 10.1109/temc .2007.913226</p> <p>Cooray, V. (2009). Propagation Effects Due to Finitely Conducting Ground on Lightning-Generated Magnetic Fields Evaluated Using Sommerfeld’s Integrals. IEEE Trans. Elec-tromagn. Compat., 51(3), 526-531. doi: 10.1109/temc.2009.2019759</p> <p>Hou, W., Zhang, Q., Zhang, J., Wang, L., & Shen, Y. (2018). A New Approximate Method for Lightning-Radiated ELF/VLF Ground Wave Propagation over Intermediate Ranges. Int. J. Antennas Propag., 2018(6), 1-10. doi: 10.1155/2018/9353294</p>

Traditional lightning detection and location networks use the time of arrival (TOA) technique to locate lightning events with a single time stamp. This contribution introduces a simulation study to lay the foundation for new lightning location concepts. Here, a novel interferometric method is studied which expands the data use and maps lightning events into an area by using coherency. The amplitude waveform bank, which consists of averaged waveforms classified by their propagation distances, is first used to test interferometric methods. Subsequently, the study is extended to individual lightning event waveforms. Both amplitude and phase coherency of the analytic signal are used here to further develop the interferometric method. To determine a single location for the lightning event and avoid interference between the ground wave and the first skywave, two solutions are proposed: (1) use a small receiver network and (2) apply an impulse response function to the recorded waveforms, which uses an impulse to represent the lightning occurrence. Both methods effectively remove the first skywave interference. This study potentially helps to identify the lightning ground wave without interference from skywaves with a long-range low frequency (LF) network. It is planned to expand the simulation work with data reflecting a variety of ionospheric and geographic scenarios.